Background: Indoles and various indolyl derivatives are very common in naturally occurring
biologically active compounds. Many methods are being developed for the synthesis of various
bioactive indole derivatives.
Objective: Synthesis of biologically promising structurally diverse indole derivatives under mild and
environmentally benign conditions.
Methods: Synthesis of 3-hydroxy-3-(5-(trifluoromethoxy)-1H-indol-3-yl)indolin-2-one was achieved
by the reaction of an equimolar mixture of isatin and 3-(trifluoromethoxy)-1H-indol using 20 mol% of
mandelic acid as catalyst in aqueous ethanol at room temperature. Under the same optimized reaction
conditions, synthesis of 3-(3-hydroxy-2-oxoindolin-3-yl)chroman-2,4-diones was accomplished via the
reactions of substituted isatins and 4-hydroxycoumarin. On the other hand, 2-hydroxy-2-(indol-3-yl)-
indene-1,3-diones and 10-hydroxy-10-(5-methoxy-1H-indol-3- yl)phenanthren-9(10H)-one were synthesized
from the reactions of indoles and ninhydrin or 9,10-phenanthrenequinone respectively using
the same 20 mol% of mandelic acid as an efficient organo-catalyst in aqueous ethanol at room temperature.
Results: Mild, safe and clean reaction profiles, energy efficiency, high atom-economy, use of naturally
occurring non-toxic organo-catalyst, easy isolation procedure by avoiding column chromatographic
purification and gram scale production are some the major advantages of this developed protocol.
Conclusion: A simple, straightforward and eco-friendly protocol has been developed for the efficient
synthesis of biologically promising novel 3-hydroxy-3-(5-(trifluoromethoxy)-1H-indol- 3-yl)indolin-2-
one, 3-(3-hydroxy-2-oxoindolin-3-yl)chroman-2,4-diones, 2-hydroxy-2-(indol-3- yl)-indene-1,3-diones
and 10-hydroxy-10-(5-methoxy-1H-indol-3-yl)phenanthren-9(10H)-one using a catalytic amount of
mandelic acid in aqueous ethanol at room temperature.